There's no real advantage to adding a stand alone DAC to the CD-23. As you mentioned the Ring-DAc was an still is one the best internal/external DAC's available. The only reason Arcam changed from Ring Dac to Wolfson was because they couldn't reach a agreement on price an parts availability with the folks who make Ring Dac.
As for the CD-33 it is not better than the CD-23(checkout Hi Fi choice review) as I mentioned above the reason Arcam switched from Ring Dac to Wolfson was a business decision not because it was better!
I have had the CD-23 and switched to the Audio Research CD-3 MkII and the difference is not that much, certainly not $3000 better. I also compared the CD-3 MkII to the Cary 303/200 again the difference was not a $2500 difference.
That alone was an eye opener for me. In addition, there was an article in stereophile not too long ago that spoke to this very issue. Basically what was said was there's not that much audible difference between middle grade digital players($2000-$5000)an the upper grade($6000-$12,000)players
Essentially they said take the difference in money and buy more CD's
I know that there're probably a lot of people who would disagree(and that's fine)but that has been my experience.
The only thing you're going to gain by going from a standalone(CD-23) to seperate DAc is needing more shelf space!
Also, digital technology has not advanced that much( in terms of audible difference)to warrant the dollar difference. Just my opinion.